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Ceramic arc tube and end plugs therefor and methods of making the sameCeramic arc tube and end plugs therefor and methods of making the same description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070035250, Ceramic arc tube and end plugs therefor and methods of making the same. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] This invention relates to a ceramic arc tube and end plugs therefor and to methods of making the same. [0002] The present invention relates to a ceramic arc chamber for a discharge lamp, such as a ceramic metal halide lamp. In particular, this invention relates to a method of manufacturing ceramic arc chambers, and more particularly, to a method for forming ceramic arc chambers. [0003] Discharge lamps produce light by ionising a fill such as a mixture of metal halides and mercury by passing an electric arc between two electrodes. The electrodes and the fill are sealed within a translucent or transparent discharge chamber which maintains the pressure of the energised fill material and allows the light emitted thereby to pass through it. The fill, also known as a "dose", emits a desired spectral energy distribution in response to being excited by the electric arc. [0004] Previously, the discharge chamber in a discharge lamp was formed from a vitreous material such as fused quartz, which was shaped into a desired chamber geometry after being heated to a softened state. Fused quartz, however, has certain disadvantages which arise from its reactive properties at high operating temperatures. For example, at temperatures greater than about 950 to 1000.degree. C., the halide fill reacts with the glass to produce silicates and silicon halides, reducing the fill constituents. Elevated temperatures also cause sodium to permeate through the quartz wall. These fill depletions cause color shift over time, which reduces the useful life of the lamp. [0005] Ceramic discharge chambers were developed to operate at high temperatures for improved color temperatures, color renderings, and luminous efficacies, while significantly reducing reactions with the fill material. U.S. Pat. Nos. 4,285,732 and 5,725,827, for example, disclose translucent polycrystalline sintered bodies where visible wavelength radiation is sufficiently able to pass through to make the body useful for use as an arc tube. [0006] Typically, ceramic discharge chambers are constructed from a number of parts extruded or die pressed from a ceramic powder and then sintered together. For example, referring now to European Patent Application No. 0587238, five ceramic parts are used to construct the discharge chamber of a metal halide lamp. Two end plugs with a central bore are fabricated by die pressing a mixture of a ceramic powder and inorganic binder. A central cylinder and the two legs are produced by extruding a ceramic powder/binder mixture through a die. After forming the part, it is typically air sintered between 900-1400.degree. C. to remove organic processing aids. Assembly of the discharge chamber requires tacking of the legs to the cylinder plugs, and the end plugs into the end of the central cylinder. This assembly is then sintered to form joints which are bonded by controlled shrinkage of the individual parts. Obviously, a simplified form of the product would be achieved by the reduction in the number of components separately formed. Moreover, the step of properly joining the compounds is time consuming, expensive and a potential point of failure. [0007] For example, the number of component parts is relatively large and introduces the corresponding number of opportunities for variation and defects. Also, the conventional discharge chamber includes four bonding regions, each of which introduces an opportunity for lamp failure by leakage of the fill material if the bond is formed improperly. Each bonding area also introduces a region of relative weakness, so that even if the bond is formed properly, the bond may break during handling or be damaged enough in handling to induce failure in operation. [0008] Another disadvantage relates to the precision with which the parts can be assembled and the resulting effect in the light quality. It is known that the light quality is dependent to a substantial extent on the voltage across the electrode gap, which in turn is dependent on the size of the gap consistently achieve the gap size within an acceptable tolerance without significant effort devoted to optimising the manufacturing process. Accordingly, it would be desirable to minimise the component parts necessary to manufacture the ceramic arc chamber. However, divergent shrinkage rates of variously shaped components and other factors have limited the ability to manufacture in a more efficient manner. [0009] A first attempt to deal with theses problems is described in U.S. Pat. No. 6,679,961. In this Patent, an arc chamber is formed of three parts, a central body member and two leg portions, one of which fits into an opposite end of the central body member and, to this end, is provided with a transition portion which fits into the central body member. This transition portion has a flange which is of the same diameter as the external diameter of the body member and thus forms a shoulder which sits against the end of the body member and locates the leg portion in relation to the body member. The leg portions are initially formed by die pressing to the appropriate external shape and then, having thereafter been heat treated to remove binder from the pressed member, are machined to provide a through bore to take the electrode and its lead in wire. [0010] Additionally, it has been found that the accuracy of the pressings described in this US Patent is insufficient for accurate assembly and extensive machining is necessary before the parts can be assembled into a suitable arc chamber. [0011] The present invention seeks to provide an arc chamber in which the leg parts can be made by pressing and which do not require machining before assembly to the main body portion of the chamber. [0012] According to the invention, a one piece end plug for use in a three part ceramic arc tube comprising a cylindrical head portion of a diameter suitable for insertion into a cylindrical body portion and a circular leg portion formed on the head portion so as to be coaxial therewith and tapering inwardly from the head portion outwards, the whole having a through aperture for receipt of electrodes and being formed of single ceramic pressing. [0013] The end plug may be formed from approximately 95% by weight ceramic material and approximately 5% by weight organic additives. The ceramic material may be 99.99% Al.sub.2O.sub.3. [0014] The ceramic material may have added to it a metal oxide such as MgO in the range of 100 to 1000 ppm. [0015] The organic additives may comprise a mixture of monomeric and polymeric alcohols, carbonic acids and ethers. [0016] The end plug may have a ratio of overall length to minimum leg outside diameter of 5.4 to 10.7, preferably 6.15 to 9.72 [0017] The end plug may have a ratio of overall length to plug outside diameter of 1.5 to 4.0, preferably 1.75 to 3.61. [0018] The end plug may have a ratio of plug diameter to minimum leg diameter of 2.4 to 4.3, preferably 2.69 to 3.94. [0019] The end plug may have a ratio of leg length to plug length of 3.8 to 8.3, preferably 4.25 to 7.5. [0020] The end plug may have a ratio of leg length to overall diameter of 0.7 to 1.0, preferably 0.81 to 0.88. [0021] The end plug may have a leg aspect ratio of 3.5 to 7.3, preferably 3.90 to 6.66. [0022] The end plug may have a plug aspect ratio of 0.2 to 0.5, preferably 0.22 to 0.48. [0023] The end plug may have a ratio of plug length to overall length of 0.1 to 0.2, preferably 0.12 to 0.19. Continue reading about Ceramic arc tube and end plugs therefor and methods of making the same... Full patent description for Ceramic arc tube and end plugs therefor and methods of making the same Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Ceramic arc tube and end plugs therefor and methods of making the same patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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